This invention relates to a process for producing compositions which contain block copolymers having low cohesive strength and low elasticity. More particularly, this invention relates to such a process wherein the high strength, highly elastic block copolymers made by coupling low strength block copolymers are decoupled during end use processing, thereby regenerating the low strength block copolymer.
It is known that a block copolymer can be obtained by an anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU A--B or A--B--A or B--A--B or (A--B).sub.n --X
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, while block B is a polymer block of a conjugated diene such as polybutadiene or polyisoprene. The proportion of the thermoplastic blocks to the elastomeric polymer block and the relative molecular weights of each of these blocks are balanced to obtain a rubber having unique performance characteristics.
A--B--A triblock polymers and (A--B).sub.n --X coupled polymers usually are very strong and elastic while A--B diblock polymers or (B--A).sub.n --X coupled polymers are weak and have little resilience or elasticity. Blends of the triblock (A--B).sub.n --X polymers and A--B diblock polymers described above are often used in various compositions which require the properties of both. For instance, in adhesives the triblock polymer is required for its strength but it is normally not tacky enough for adhesive uses so a diblock is blended in because diblocks are generally characterized in that they are very tacky. Similarly, blends of this type are used in the impact modification of nylon/polyphenylene ether blends and also in polyolefin blends to reduce viscosity and retain impact strength.
Blends of these strong and weak polymers which have high amounts (e.g., greater than 50%) of weak polymer are very soft and tend to block (undesirable cohesion of the polymer--it tends to fuse into solid blocks under normal storage conditions) severely. In addition, such polymers are difficult to manufacture, ship, and handle in subsequent compounding. The A--B--A triblock and (A--B).sub.n --X coupled polymers alone do not suffer from these disadvantages. They are hard, resist blocking, are relatively easy to manufacture, ship, and use. Thus, it can be seen that there is a need for a way be able to have the advantages of the weak AB diblocks in end use compositions without having to manufacture and handle them. The present invention provides a way to achieve that goal.